After 2 years of good use, left crank arm on my FC M542 crank set has started to get loose quite often (cca 500 km). It is usually indicated first by few now an then click-clanks somewhere in the region around BB. When checked, the bolts are loose, and the safety plate loses it's "nose"

As far as I can see there are 2 visible problems:

The spline on the crank arm shows some wear, but they do not seem completely damaged.

One of the fixing bolts is not original Shimano one M6x19, but M6x20 from hardware store with filed head so that it can be inserted in crank arm. It is also missing washer.

I intend to buy original bolt with washer. Other than that, is there something I can do to delay changing the whole crank set?

Maybe using some threadlocker like loctite 242? Where would I apply it on the bolts or on the spline? If it is applied to the bolts, does that mean that I have to use more force while tightening them, or just use force as usual?

The main thing is to make sure the bolts are good and tight. But, unfortunately, if a crank arm is run loose it will develop a "flare" and no longer fit tightly on the shaft. This is especially bad on the left side as "precession" will tend to loosen the bolt on that side, if there's any looseness in the crank splines at all. Loctite on the bolt will help but is not a panacea for a flared crank arm. Be sure use a removable type. It will not make it substantially harder to install the bolt, but will make it significantly harder to remove.
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Daniel R HicksJan 23 '13 at 16:23

I've always figured you could make a shim from an aluminum can for square shafts, but I don't think you can make that work very well for a splined shaft.
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Daniel R HicksJan 24 '13 at 12:01

3 Answers
3

Thread lock will help hold the bolts on. I would apply it to the bolt thread, and not the actual crank arm thread. You will need more force to remove them when you do come to remove the crankset. But they shouldn't need any more strength to put them on. Just tighten them to the recommended torque setting (it should have it written on the crank arm).

Fundamentally though, it sounds like you have a few problems which only a new crankset and BB would solve.

I got a Tiagra Hollowtech chainset on ebay for around £30-£40. It might cause more harm than good trying to threadlock it all together, and give you more problems when trying to get it off and replace it later down the line.

Get the proper bolt and make sure it's tensioned to the proper torque. If you're still having problems with the crankarm coming loose then there's probably another issue.

Typically if you have a crank of that style that wont stay tight it's due to the pinch bolts not being tight enough and allowing the crankarm to come loose at some point in the past. That can damage the interface and cause the crankarm to come loose from the spindle on a regular basis.

If using the right hardware and torque doesn't solve your issues, take the bike to a reputable shop for a once over.

Ok here is what I have done, and how it all went. I bought a pair of new proper bolts with washers. With shipping they were two most expensive bolts I have bought in my life, around $10. I have cleaned everything well, installed new bolts and tightened them as good as I could, by alternating bolts for each half turn. After that for few days I kept checking them and tightening them when needed.

After that one year passed and I have made more than 10.000km on that crank set. When the bottom bracket was changed at about half year ago, I told to the guys in bike shop to be extra careful, and I have followed on them by checking and retightening bolts for few days, and it all went well.

A few weeks ago I have sent the bike to the service and forgot to:
- mention to tighten the bolts really tight
- follow up on them and check for few days if bolts are tight.

So when I have returned from one night ride, I have noticed that crank arm has almost fell of, and now it was beyond repair.

So to summarize, replacement bolts turned out to be a good investment, maybe it could have been even better if I was more careful.